The disclosure is related to method and apparatus for improving the manufacturing costs and the reduction of emissions to the air in the preparation of fuels from biomass.
Pellets produced from biomass are increasing with regard to production of thermal electricity as replacement for coal and is thus a contributor to reduction of CO2 emissions. In recent years, pellet production has been dominated by production without heat treatment of the biomass, thus producing so-called first generation pellets or “white pellets”. Now focus is changing production of so-called second generation pellets where the biomass is heat treated to change the properties of the biomass. One of these methods utilizes so-called steam explosion method, where biomass is heat treated with steam.
In pellets production which uses steam explosion method as method for heat treatment, such as described in Patent 320 971, or by other related methods of heat treating biomasses or so called ligno-cellulosic material, these methods involve emissions to air which have not been considered adequately. The steam explosion method for pressurizing a container of the supplied biomass and then pressurizing with steam supply, with following instantaneous discharge, also provides a discharge of steam and volatile gases from the mass having being heated.
There are also existing and expired patents that deal with steam treatment of biomass or so-called lignocellulosic materials. These mainly conclude that it is advantageous to apply steam (saturated or superheated steam) to a closed container I which biomass have already been supplied and heating it to a given temperature in the temperature range from 160 degrees up to 300 degrees, depending on the what you want to achieve in the reactor, and then emptying the reactor in one or two steps.
Prior art teaches to vary the degree of filling of the reactor, to vary moisture of supplied biomass, and to calculate the associated, required amount of steam to both heat the biomass (dry material+moisture in the mass) and to create the desired pressure/temperature relationship in the reactor. Typical operation ranges are between 150 and 280 degrees Celsius, but it turns out that the preferred properties for energy purposes are best achieved if the temperature is held between 190 and 235 degrees Celsius, or approximately a pressure from 15 to 28 bars.
When heating the moist biomass in a pressure vessel in which steam is supplied, the steam will condense on the particles to transfer energy to the biomass, and its moisture is heated to the desired temperature range. In addition an amount of steam has to be supplied to achieve the desired total pressure and temperature of the atmosphere surrounding the biomass.
A challenge with this system is that it uses quite a lot of energy to produce the required amount of steam (in the order of 200-600 kg steam per ton of material). A certain amount of biomass is supplied to which the amount of steam to be added is determined as a function of the filling level of the reactor, of the desired pressure and temperature, of the inlet temperature and of the moisture level of the mass to be treated. When there is little mass volume in a reactor, less steam is required to be heated than when more mass is present in the same container/reactor, and drier mass requires less steam to be heated than a wetter material, while the desired pressure/processing temperature will subsequently affect the total steam demand.
Emptying the reactor can be done by emptying in one or more steps, as described in Norwegian patent No. 320,971, Canadian Patent No. 1,267,407 (De Long) or others. One can thus reduce the so-called expulsion pressure to a lower level than the desired operation pressure. This expulsion pressure can be from 1-3 bars up to the processing pressure depending on what is actually desired to achieve. If it is just for emptying the reactor, a lower expulsion pressure is desirable, and if a defibration or a “bursting” of the fibers is desired, a higher expulsion pressure is desired, i.e. greater pressure difference between the reactor and the site to which the mass is discharged (often close to atmospheric pressure or slightly higher in order to reduce the volume).
Emptying/discharge of biomass from a reactor can proceed in the form of a flow through a pipe or passage, expanding towards a volume of lower pressure, where the mass is separated from the steam, so that the mass remains in the tank/separator/cyclone while the steam expands out in the open.
Emptying occurs rapidly, the driving force being the pressure difference. The greater the pressure difference the greater the amount of steam emitted simultaneously with the mass to be used further. When this occurs a large amount of energy is thus released. This energy should preferably be recovered.
During the heat treatment volatile gases (volatiles) are released from the biomass and are mixed with steam and contaminate the steam. The gases produced are mainly organic acids and aldehydes, which are discharged and produced with time. The amount of gas depends on time, temperature and pressure. The primary and predominant initial reaction is the decomposition of hemicellulose to, for example, furfural, formic acid, acetic acid. A plethora of gas components have been observed in the mixture. These gases have different boiling points and are either soluble in water or insoluble in water at different temperature ranges. Several of these gases have a strong odor that is characteristic of the method and many find the smell unpleasant, and it also contains a lot of carbon remains that should rather be reused.
The general problem with heat recovery associated with this type of process is that a large amount of gas and steam are discharged within a few seconds, why there are high demands on heat exchange unit, and in addition the product flow is very complex where volatile (non-condensable) and condensable gases come with a little predictable mass composition. This can lead to a build up of pressure downstream of the process that interferes with the mass flow. In addition to this comes the fact that many of the components are crude in the sense that they have a strong odor and can lead to physical discomfort for personnel who are exposed to them.